1. Field of the Invention
This invention relates to a fusing apparatus as is commonly used in xerographic copying machines, and more particularly to a fusing apparatus including a heated roll fuser member in pressure contact with an endless belt so as to form an area through which a substrate carrying a toner image thereon can pass to fuse the toner image to the substrate.
2. Description of the Prior Art
In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, which areas correspond to the graphic material on the original document. The resulting electrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner). The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is cleaned and the photoconductor is reused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. Since the developing material is heat fusible, application of sufficient heat to the paper causes the developing material to melt and be fused into the paper so as to be permanently affixed thereto.
One basic and advantageous approach to fusing in a xerographic copying machine is the use of the so-called hot roll pressure fusing apparatus in which toner particles are melted by being in the direct contact, under pressure, with a hot surface of a heated fuser roll. However, this type of apparatus does present certain problems. For example, as the toner particles are heated they soften and become sticky to the extent that they can readily adhere to other surfaces. During a typical fusing operation there is a tendency for part of the heated image to stick to the heated fuser roll. The toner which adheres to the fuser roll will, of course, transfer to the next sheet of support material passing through the fuser, thus producing dirty copies. This process is commonly referred to in the printing art as "offset".
In an attempt to avoid the problem of offset, heated fusing rolls were developed which use either (1) a thin outer layer of a tetrafluoroethylene resin (e.g. a tetrafluoroethylene resin sold under the trademark "Teflon" by E. I. DuPont de Nemours & Co.) and a silicone oil film on the resin, or (2) a thin outer layer of a silicone elastomeric material (silicone rubber) and a silicone oil film on the elastomeric material. The physical characteristics of the Teflon material or the silicone rubber with the silicone oil film are such that they are repellent to sticky or tacky substances. Although the use of either of the above-described coated heated fuser rolls or many other different types of coatings used to prevent offset have helped to prevent this problem, they have not completely solved it.